The present invention relates to an etching method.
To keep pace with the great improvements made with respect to the degree of integration of semiconductor devices in recent years, miniaturization of various elements formed on semiconductor substrates has become one of the crucial technical requirements. In order to meet this requirement, the distance between the individual gates (electrodes) formed on a semiconductor substrate must be reduced. If contact holes are formed between the gates, the contact holes, too, be miniaturized. However, as the distances between the gates become increasingly small, the difficulty in forming even smaller contact holes at accurate positions increases, due to the limits of the alignment performance of the stepper and the like. Accordingly, a self-aligning contact technology, whereby self-aligned contact holes are formed within the minute space between individual gates by forming a protective film (base) constituted of, for instance, an SiNx film (silicon nitride film) at the surfaces of the gates and thereby preventing the gates from becoming etched during the contact hole formation, has been proposed of late.
For the etching process implemented to form the contact holes and, more specifically, during the etching process implemented to form contact holes passing through the insulating film covering the individual gates and constituted of, for instance, an SiO2 film (silicon oxide film) between the gates, a mixed gas constituted by adding CO to C4F8, for instance, is used as the processing gas. gas constituted by adding CO to C4F8 provides advantages in that an almost vertical etching shape is achieved and in that the selection ratio of the SiO2 film to the SiNx film (SiO2 film etching rate /SiNx film etching rate) (hereafter referred to as the xe2x80x9cselection ratioxe2x80x9d) is improved.
However, if C4F8, which is not readily degraded in the atmosphere, is directly released into the atmosphere without having been dissociated during the processing, it becomes a greenhouse effect inducing gas to hasten the process of global warming.
For instance, it has been reported that the life of C4F8 in the atmosphere is 3200 years (Climate Change 1995, the Current Status and Measures Taken with Respect to the PFC Problem At Semiconductor Mass Producing Plants), whereas the life of C5F8 in the atmosphere is only one year.
An object of the present invention, which has been completed by addressing the problems of the prior art discussed above, is to provide a new and improved etching method that allows an etching process to be implemented using a gas that becomes degraded in a relatively short period of time when it is released into the atmosphere and does not induce the greenhouse effect while achieving vertical etching of an SiO2 film and a selection ratio which are comparable to or better than those in the prior art.
In order to achieve the object described above, in the etching method according to the present invention, in which an SiO2 film formed on a workpiece placed within an airtight processing chamber is etched by raising a processing gas induced into the processing chamber to plasma, the processing gas contains at least C5F8 and O2.
It has been confirmed that C5F8 constituting the processing gas according to the present invention becomes degraded in a relatively short period of time in the atmosphere compared to the CF gases that are used as processing gases in the prior art such as CF4, C2F6 and C4F8. Thus, the use of C5F8, which does not induce the greenhouse effect even when it is directly released into the atmosphere, will contribute toward prevention of global warming.
In addition, since C5F8 is relatively rich in carbon compared to the CF gases such as CF4, C2F6 and C4F8 mentioned above, a carbon containing film that is to constitute a protective film can be formed with ease at the etching mask, at the shoulders of the etching mask pattern or at the inner side walls of the contact holes. However, if C5F8 alone or a mixed gas constituted of C5F8 and Ar is used for the etching process, the carbon-containing film remains at the bottoms of the contact holes to result in a so-called etch stop. Accordingly, the processing gas according to the present invention contains O2 so that the quantity of carbon-containing film within the contact holes can be controlled through the O2 to prevent the occurrence of an etch stop and to achieve control of the contact hole angle. As a result, compared to the prior art, which uses a gas containing C4F8 and CO for the etching process, the contact holes achieving an equal or higher degree of verticality can be formed.
Furthermore, forming an SiO2 film over an SiNx film formed on the workpiece prevents etching of the exposed surface of the SiNx film by the carbon-containing film covering the exposed surface to achieve an improvement in the selection ratio.
Also, by setting the flow rate ratio of C5F8 and O2 within the range of 1xe2x89xa6(C5F8 flow rate/O2 flow rate)xe2x89xa61.5, a high selection ratio comparable to or better than the selection ratios achieved when using the processing gases in the prior art mentioned above is achieved.
Moreover, a nearly vertical etching shape can be obtained by, for instance, setting the flow rate ratio of C5F8 and O2 within the range of 1.3xe2x89xa6(C5F8 flow rate/O2 flow rate)xe2x89xa61.625, by setting the atmosphere pressure within the processing chamber to 45 mTorrxcx9c50 mTorr, by setting the temperature of the stage on which the workpiece is mounted within the range of 20xc2x0 C.xcx9c40xc2x0 C. or by adding an inert gas such as Ar to the processing gas.